Smoker blueprint

Hank’s Offset Smoker Manifesto

So, you plan to build an offset smoker? Congrats! It’s a large undertaking, but fun and educational. This page is thought to be your guide when building your own offset smoker.

Background

I’m an advocate for what is commonly known as ‘fat-stack’ smokers. Being an engineer means calculating and measuring, instead of just guessing. A smoker needs to breathe properly. Below follows a detailed walkthrough of all parts of a smoker, regardless of whether it is a normal one or a reverse flow smoker. I discuss what you need to think of, and also what can be ignored. My aim is to take away some of the guesswork and myths when it comes to building offset smokers. This is my manifesto.

How it works

An offset smoker is typically using wood as a fuel source. ‘Offset’ means the fire is offset, typically sideways, so you get the indirect heat setup that is key to barbecue.

There are roughly two types of offset smokers.

Normal offset smoker

Normal Flow Offset Smoker
Normal Flow Offset Smoker

As you can see from the illustration above, the fire is in the firebox to the right. Hot air and smoke travels through the food chamber and out through the smoke stack. This is a ‘simple’ design that works well. Depending on how it is constructed it naturally runs hotter in the firebox end of the food chamber but this can be compensated for. If you don’t feel like building one but instead want to buy one then Hank’s True BBQ is manufacturing top quality pits. Take a look here!

Reverse Flow Offset Smoker

Reverse Flow Offset Smoker
Reverse Flow Offset Smoker

As illustrated the reverse flow smoker uses a long baffle to move air all the way through the food chamber before being turned around, or reversed. This helps even out temperature differences since the radiant heat from the baffle and food chamber walls heat up the air more evenly. The drawback is that air must travel longer, thereby potentially reducing draft if it isn’t dimensioned properly. The most obvious sign of a reverse flow offset smoker is the chimney being mounted on the same side as the firebox. If you don’t feel like building one but instead want to buy one then Hank’s True BBQ is manufacturing top quality pits. Take a look here!

Reverse Flow Offset Smoker – a different version

Reverse Flow Offset Smoker with perforated baffle
Reverse Flow Offset Smoker with perforated baffle

The main idea behind reverse flow is (as mentioned) to reduce temperature differences in the food chamber. Completely depending on different constructions an offset smoker is typically hotter the closer to the fire you get. A common way to solve this is to use a slanting baffle perforated with different size holes. See picture above. The hot air flow is slightly redirected by the smaller holes at the top, forcing some of the hot air down where the larger holes are. This distributes heat more evenly when it enters the food chamber. One must remember that air moves a lot faster through an offset smoker than in your ordinary kettle or kamado.

If unsure of which to build, start with a normal offset smoker, due to a simpler construction. That being said the reverse flow design suits smaller smokers better since the BTU (energy) in is wood high. You get slightly more even temps with the reverse flow in a small smoker.

NOTE: many first time builders I talk to have their mind set on building a reverse flow smoker since they just ‘know’ it is better. And that without even having used one, reverse flow or not. I think that is largely due to the many cheap offset smokers available on the market, which are badly designed and have large temp swings. A well constructed offset smoker has no issues with temp swings. My offset smoker ‘Rude Boy’ is a normal flow smoker, and I have a 5-8 degree temp variance in the food chamber. That is way less than most reverse flow smokers commercially available.

The Food Chamber

Start with the food chamber and calculate the rest of the smoker accordingly. Size and shape is governed mainly by two things:

  1. Capacity. How much food do you need to cook at once?
  2. Appearance. What style of smoker are you looking for?

Several grates at different levels are very common, as well as different door mechanisms. I usually measure briskets and ribs with a folding rule to be able to estimate shelf size. Make sure you measure height also to get a correct vertical distance between shelves. Regardless of size, two shelves/grates are recommended. This is to get more cooking capacity, but the temps will also be a little different (hotter on the higher grates), and this can be advantageous when smoking different cuts of meat at the same time.

Offset Smoker Food Chamber Shapes
Offset Smoker Food Chamber Shapes. The round one is most common.

The Firebox

The firebox needs good oxygen supply, and should have room for building a good fire with 5-6 logs. The purpose of the firebox is to create a fire that can warm up and hold temp in the food chamber. The size of the firebox is not related to the size of the food chamber. Why not, you may ask? Well, the heat energy in burning wood is so high that even a 4-5 log fire can sustain temps in a 1000 gallon smoker. There are various suggestions online to make the firebox a certain percentage size of the food chamber, but that simply doesn’t hold true. Take a look at really large smokers like the ones Aaron Franklin buids, and there you will see the firebox is relatively small.

Firebox Size

In general, the firebox can be smaller than you think. Pure wood has higher BTU (heat energy) than charcoal or briquettes, so even a small fire will produce enough heat. When I build a firebox I make a cardboard box as a model first. It saves a lot of time and measurements. If unsure, start with a firebox that is 50 cm wide, 50 cm tall, 60 cm deep (20″ wide, 20″ tall, 24″ deep). That size will suit most backyard size smokers. The reason for having it slightly longer is to allow for it to protrude into the firechamber, but also to be able to build your fire further in (common during winter) or closer to the door.

Is there such a thing as too small a fire box? Yes if the fire box is too small you will have issues with temp spikes in the food chamber. A certain size fire box gives you a bigger thermal mass (bigger fire box -> more metal), and also a larger buffer of hot air. Hence the general suggestion on size in the previous paragraph.

Cardboard Firebox
Cardboard Firebox

Smoke Stack Size

In order for an offset smoker to work well you need a clean burning fire. A clean burning fire requires good draught, which is accomplished with an appropriately sized smoke stack. Many commercial smokers have a smoke stack that is too thin and/or too short if you ask me. The smoke stack is the engine in your smoker. By draught I mean the amount of air being pulled through the smoker, measured in ft3/s, or m3/s if you use the metric system.

A common misconception online is that the smoke stack should be sized according to the size of the food chamber or fire box. This is wrong. Draft is primarily controlled by diameter and length of smoke stack – within reasonable limits, see below.

The size of the smoke stack is governed by how much natural draught you want. Natural draught is controlled by the temperature difference between the food chamber and the outside air. That holds true within reasonable limits.

What do I mean by that? Well, imagine a correctly dimensioned smoke stack for a 500 gallon smoker measures 10″ in diameter and is 3 meters tall. Now put that on a backyard size smoker – would that still produce the same draft? No, because the volume of the smoke stack is so much larger in relation to the food chamber. In this scenario the smoke stack needs to be scaled down proportionally.

Smoke Stack Placement

Most smoke stacks are located at the very top of one end of the food chamber. Some are centered vertically.

Smoke Stack Placement
Smoke Stack Placement: top, middle and bottom.

Placement of the smoke stack is more important than you think. In an offset smoker there’s a lot of hot air moving through at a constant pace. Hot air rises, meaning it will move up to the top of the food chamber as it moves across. By placing the smoke stack lower down on the food chamber, the hot air flow will be temporarily reduced (when forced down), causing some of the hot air to transfer its heat back into the food chamber and therefore reduce the temperature differences slightly. You can place the smoke stack even lower than that if you want to, but in my experience it doesn’t help. See the image below which illustrates this back pressure.

For reverse flow smokers the smoke stack is mounted high. Due to air traveling longer in the food chamber, you want to improve draft by making it as easy as possible to get air from A to B. Also the effective height of the food chamber is reduced with the baffle in place.

Smoke stack placement
Smoke stack placement

Smoke stack transition (elbow)

The aerodynamically oriented may wonder if the elbow or bend should be as smooth as possible to allow the smoker to breathe fully.

Different ways of constructing the smoke stack elbow
Different ways of constructing the smoke stack elbow

You can use a 90 degree bend, or ‘soften’ it a bit in various ways. Does it really matter? A 90 degree bend will affect draught slightly, but the effect is so small it can be ignored in this type of application. An offset smoker has a lot higher air flow than a kettle grill, but at the same time it’s not a Formula 1 car either. Choose what is easy to build or that matches the look you’re after. I bought a smoothly bent (1/4 circle) pipe myself, it was the easiest way for me.

Materials – what to use

Regular steel is by far the most common material. It is generally available, and affordable. Stainless works just as well, but comes with a much higher price tag. The one material to steer clear of is galvanized steel. it contains a lot of zink (a natural part of the galvanization process), and it releases toxic gases when heated. Some may counter by saying that the temp in the food chamber is too low for the zink to evaporate, but the fire temperature in the fire box is a lot higher than that, so avoid galvanized steel at all times.

General construction ideas

The opening from the firebox into the food chamber should be as large as, or bigger than, the smoke stack size. This is to avoid throttling the air flow. So: if you ‘walk’ the path the air takes, but backwards, from the smoke stack to the firebox, each opening/aperture the air passes should be equal to, or bigger than, the previous opening/aperture.

How to manage your smoker fire

26 thoughts on “Hank’s Offset Smoker Manifesto”

  1. Thanks, this is great information and is not exactly what I have been finding other places. I’m in the process of making my first offset smoker and this information will go directly into my design.

  2. Thanks for these explanations. How long should the stack be? How did you calculate that? If the stack is too long, the air is cooled down and condensed. The cold air presses back into the chamber and the draft is disturbed. Or did I misunderstand something? Greetings from Germany…

    1. That depends 🙂 If the stack is too long, the draft is too strong, meaning you get zero smoke flavor. If it is too short, the fire doesn’t burn clean enough, and you get bad smoke flavor. The trick is finding a good average. I use natural draft calculations (thermodynamics) to get close enough.

    2. Brilliant manifesto! You’ve confirmed a few of my assumptions and have given me a lot to think about on things i didn’t consider.

      The dimensions is one of my biggest worries. What I’m planning is:
      – 6.35mm thick steel

      – 1 meter wide, 45cm deep cook chamber

      – 50cm wide, 35cm firebox

      – 10cm diameter smoke stack

      – smokestack will go centered on the side at grate

      – the door to the firebox will have an air intake slide only at the bottom

      I noticed the dimensions of the firebox you talked about is about the same as what I’m planning and you say the size for the firebox isn’t relevant to the size of the cook chamber. So does this work?

      My biggest question is, how tall should i build the smoke stack? You said the size of the smoke stack friends on the amount of drought i want. So how much should I want for an ideal cook/smoke? Or should I make it bigger than need and use a damper?

      Thanks again!!

      1. Cool! The dimensions overall look good. I would make the firebox deeper, 35 cm is just barely enough to fit an ordinary log. Make it 50 cm deep instead. Make the smoke stack 80 cm tall, and you’re good.

  3. I’m in the beginning phase of designing my smoker. I live north of Denver (6k ft) so I naturally have to take into account getting enough oxygen into the smoker to maintain a steady heat.

    My question is if you have a design that works better/best at elevation?

    1. Hi Nate! No, I don’t have a specific design. The ‘regular’ design works. What I do (as you may have seen) is that I run with a smoke stack with no vents. I don’t have a vent on the firebox either, I run it with the door open. That way you’re free to build the fire the way you want, and therefore also take into account high altitude.

  4. Hi Hank, thanks for all the info! Does the wall thickness of the smoke stack matter? I am struggling to find some tubing of the right size. Right now, I am looking at a piece of tubing in these dimensions — 4″ OD x 0.065″ Wall x 3.87″ ID Stainless Round Tube 304 Welded

    I am concerned this will be too thin

    1. Hi Gino! If you can, try to find thicker material. The tube you found is so thin it will dent rather easily. But it will work, no problem, meaning it will create the draft.

  5. Hi I’ve just built my first offset cook camber is 54”x19” old compressor tank my fire box is 18”x18”x 24” I’m currently having problems with the draft from my stack it’s currently 46” long made out off 80x80x5mm box section but am looking to modify this to some 6” pipe what length would to recommend as and average

    Cheers

    1. Hi Scott! In what way are you having problems with the draft? What does the opening from firebox to food chamber look like? Are you running with charcoal or wood? Do you have a vent in the firebox door, or is the door open? Drop me a line: henrik@hankstruebbq.com, feel free to include a few photos.

      Thanks, Henrik

  6. Hank im building a offset smoker the cooking chamber 22 by 60 im using a 24 inchby 18 firebox and a 4 inch by 34 inch pipe for a exaust vent.the cooking chamber will to firebox vent half moon with a 13.5 diameter.what do you think of these measurements and how much angle should i angle the radiant plate inside the pit
    over the fire box .please tell me what you think

  7. Have an offset smoker (0.25 in construction) with a cooking chamber of 20″X40″ and the smoke stack is 16″ from top of the 90º elbow, 0.25 inch tube with an inside diameter of 5.25 inches. But my cooking chamber opening to the stack is only 4.25 inches. Should I make the opening as wide as the stack diameter and increase my stack height by another 16 inches to improve the draught? The fire box is 18″X20″ with a 4.25″X8.50″ opening to the cook chamber.

    1. Hi Jorge! I would say yes. The opening, going backwards, should be equal to the smoke stack size or larger. That way the smoke stack is the controlling factor. And your stack is a bit short, 16″, so double it.

      Cheers, Henrik

  8. Hello Henrik,

    Thanks a lot for all the information and recipes too, I really appreciate the knowledge I got from your website, and Videos.

    I have questions regarding the design of the traditional smoker, does the size of the opening between the Firebox and the cooking chamber relate to the size of anything else? in other words how to decide the size of this opening?
    I am after an even heat distribution across the smoker, so if I add a deflector (water Pan shelf similar to the one in Franklin commercial Pit) would it help achieve that?

    Thanks a lot

    1. Hi Ahmed,

      and thanks! Yes, that opening should be bigger than the area of the smoke stack. You want the smoke stack to be the one factor controlling draft. And yes, adding a water pan will control air flow. Hot air rises, and the water pan blocks the upwards air flow.

      1. Thanks a lot, Hank that is very helpful, I have a friend who is building me an offset with the following measurement
        square firebox H 60 x W 76 x L 76
        Cooking Chamber is 76 cm Diameter x 150 cm in length
        I have 2 questions if you don’t mind, please.
        my friend suggests the smoke stake to be 17.5 cm diameter x 104 cm length (from the bottom of the smoke collector to the top of the stake), is it ok? I feel it should be taller, what do you think?
        Also, the stake’s length should be measured above the Cooking champer or the actual length from the collector to the top of the stake?

        Thanks again

        1. Hi,

          the smoke stack should be measured from collector to the top of the stack. The collector is usually small enough (height-wise) to be ignored. The 104 cm length sounds ok to me given the large diameter.

          Cheers, Henrik

  9. Hi, how are you I love all the information on this site, I’m praying to build me offset smoker, 45″ cooling chamber and 20″ firebox , my question how big should a make the cooling chamber door.

    1. Hi! That’s up to you, there’s no fixed rule. Make it conveniently large. If you plan on building slide out racks then account for that.

  10. Hey there,
    Making an offset smoker with a 20″ x 42″ cooking chamber, 20″ x 16″ firebox(1/3 size of cooking chamber). I have used a BBQ calculator I found online. The calculator suggests a 34″stack if I use 3″ pipe or a 20″ stack if I use a 4″ pipe, both which come to 5% of the firebox volume. Does this seem right? Also suggested were a 40 sq.in. vent in the fire box and a 40 sq.in. firebox to cooking chamber opening? Please help, first build.

    1. Hi Zac! Thanks for writing. Unfortunately I get a lot of these requests, and I just don’t have the time to answer them all. I would advise you to check out some commercial smokers with good reviews and try to copy/mimic their dimensions. Sorry for not helping out, I hope you understand. Anyway, I hope you build that smoker and enjoy cooking on it!

  11. I’m sorry, but this is wrong. You’ve said the stack size is not related to the cook chamber or firebox size because natural draft is governed by the temperature differential between the cook chamber and the outside air. This is generically true. However, as an engineer, surely you’re aware of the effect of placing a larger stack on a smaller cook chamber. If I were to mount the stack from my 250 gallon an my tiny backyard smoker the stack would move the same volume of air but since the cook chamber is much smaller it dramatically increases the velocity of that same volume of air. This would burn the food inside. The stack does need to be sized appropriately to the cook chamber. The same goes with the firebox, as you stated, you don’t want to throttle airflow, this means your firebox must be large enough to house a clean burning fire but also have a large enough opening so as to not restrict airflow. The firebox also needs to be sized appropriately so as to house the correct volume of air necessary to carry the heat and smoke through the cook chamber. An undersized firebox will result in airflow problems. An oversized firebox will result in a pit that has temp spikes. What you said about walking the path the air takes backwards and having each opening at least it’s large as the one preceding is true for a standard flow but not a reverse flow. The baffle plate gap does NOT need to be as large as the exhaust opening. Furthermore, on a reverse flow you want the stack mounted high, not at grate level, and the firebox mounted low. I could go on but I just feel like this misinformation could lead folks to waste their time and money building a pit that doesn’t function properly.

    1. Hi, and thanks for taking the time to leave a comment. This is some really good and detailed feedback, I appreciate that. To paint a bigger picture I wrote this a while back because I was frustrated by the many times wildly different advice and recommendations available online, including so called calculators which was an interpolation of vastly different size smokers. But in all fairness this page and its content could do with some updates, as well as some more nuances around certain details. I have done that now, and will keep updating it in the next few days.

      Thanks, Henrik

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